1. Field of the Invention
This invention relates generally to a method and apparatus for electromagnetically measuring the inside diameter of a cylindrical structure and particularly relates to an electronic inside pipe caliper. More particularly, the invention relates to a method and apparatus for logging the inside diameter of oil and gas well tubular members, e.g., casing.
2. Description of the Prior Art
It is frequently necessary to measure the inside diameter of a pipe. This is particularly true of the casing of an oil well or of a string of tubing disposed within an oil well, for example, to extract oil or gas from the well.
Such metallic pipes are buried in the ground and, therefore, are not accessible for measurement. Both the inside and the outside of such metallic pipes is subject to corrosion damage. The corrosion may be caused by injected fluids or by corrosive fluids within the ground. Besides, the pipe may be subjected to internal wear from the rods of pumps or from wear due to wire lines which may be sent into the well. Also, wear may be caused by drilling or fishing operations utilizing a drill pipe.
Measuring the damage of a pipe due to corrosion or wear is important in order to apply protective measures or to repair or replace the pipe when appropriate.
Of course, it should be realized that the internal diameter of the pipe in an oil well may vary due to permissible deviations of the wall thickness and of the nominal pipe diameter.
Because there is a real need to determine the inside diameter of buried metallic pipes, various instruments have been developed in the past for this purpose. Most of these instruments utilize mechanical feelers which are urged by spring means against the inner wall of the pipe. When one of the feelers is deflected due to the existence of pits, cracks, and other effects of corrosion or erosion of the pipe, an electric signal may be generated which is utilized to record such deviation. This may, for example, be effected by associating a magnet with the feelers. The magnet is rotated sequentially past the various feelers so that a deflection of the feeler changes the magnetic reluctance of the magnetic path which, in turn, induces an electrical signal that can be recorded. Alternatively, a potentiometer may be associated with a feeler for generating a signal proportional to the deflection of the feeler.
Some of these mechanical feeler calipers, utilize two sets of feelers, one for measuring the maximum penetration of the feeler, while the other continuously measures the average penetration of the feelers.
However, a pipe caliper of the type having mechanical feelers has a number of disadvantages. For example, such an instrument cannot detect longitudinal splits or cracks of the pipe. The pipe may be covered with paraffin, scale or other non-metallic material which may cover up faults in the pipe. The feeler, of course, cannot distinguish between a metallic pipe and non-metallic material which may cover the pipe. Therefore, certain pits covered by hardened hydrocarbons and the like cannot be detected.
Finally, each of the many feelers may scratch the pipe or scratch off the protective coating of the pipe. Even if the pipe has not been covered with a protective coating, the scratches caused by the feelers may initiate further corrosion. Consequently, the mechanical feeler-type instrument may cause change to the pipe inside diameter it is intended to measure.
U.S. Pat. No. 3,417,325 issued to McCullough and Stroud discloses a purely electrical caliper for determining the inside diameter of a metallic pipe in which a transmitter coil and receiver coil are supported on a cylindrical housing of a sonde adapted for movement through oil well casings and the like. The transmitter is excited with a.c. current of the order of 10 to 50 kilocycles per second. The voltage induced in the receiver coil is taken as a measure of the average inside diameter of the pipe being measured.
It has been found in practice, however, that the apparatus disclosed in U.S. Pat. No. 3,417,325 yields erroneous results where either the conductivity, or permeability or both conductivity and permeability of the pipe being measured vary. In actual oil field practice the conductivity of steel tubular goods varies from a range of 2.6.times.10.sup.6 mhos/meter to 7.8.times.10.sup.6 mhos/meter. Also, as is well known, the permeability of oil field tubular goods varies significantly due to the mechanical shocks, heat treating, etc. to which the goods are subjected.
Thus, it is an object of this invention to provide an inside pipe caliper which operates on electrical principles which will overcome the disadvantages of prior instruments.
Another object of the invention is to provide an inside pipe caliper which generates a signal representative of the inside diameter of the pipe which is substantially insensitive to variations of conductivity and permeability of the pipe.
Another object of this invention is to provide well logging apparatus for logging borehole casing or tubing which yields an accurate log of the inside diameter of the casing versus depth, regardless of variations of casing conductivity or permeability.